What is the purpose of __table_args__ in sqlalchemy?

Question

I have no experience with sqlalchemy, and I have the following code:

class ForecastBedSetting(Base):
    __tablename__ = 'forecast_bed_settings'
    __table_args__ = (
        Index('idx_forecast_bed_settings_forecast_id_property_id_beds',
              'forecast_id', 'property_id',
              'beds'),
    )

    forecast_id = Column(ForeignKey(u'forecasts.id'), nullable=False)
    property_id = Column(ForeignKey(u'properties.id'), nullable=False, index=True)
    # more definition of columns

Although I have checked this, I cannot understand what is the purpose of __table_args__, so I have no clue what this line is doing:

__table_args__ = (
    Index('idx_forecast_bed_settings_forecast_id_property_id_beds',
    'forecast_id', 'property_id',
    'beds'),
)

Could somebody please explain me what is the purpose of __table_args__, and what the previous piece of code is doing.

Answer 1

This attribute accommodates both positional as well as keyword arguments that are normally sent to the Table constructor.

During construction of a declarative model class – ForecastBedSetting in your case – the metaclass that comes with Base creates an instance of Table. The __table_args__ attribute allows passing extra arguments to that Table. The created table is accessible through

ForecastBedSetting.__table__

The code defines an index inline, passing the Index instance to the created Table. The index uses string names to identify columns, so without being passed to a table SQLAlchemy could not know what table it belongs to.